Year 12 Chemistry Module 7 · Organic Chemistry ⏱ ~45 min 5 MC · 3 Short Answer Lesson 5 of 23 Core Core

Hydrocarbon Reactions, Combustion, Substitution, Addition & Polymerisation

Unlock the logic behind four key hydrocarbon reactions, learn why alkanes substitute, alkenes add, all hydrocarbons burn, and how monomers chain into polymers.

Today's hook: Two colourless gases are bubbled into bromine water, one tube stays orange and the other instantly turns clear. What is different about the bonding in each gas?

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Connect: classify the hydrocarbon reaction families

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application practice Master Mastery challenge Exam Exam-style questions

Before You Read

Two colourless gases are bubbled into separate test tubes of bromine water. One tube stays orange. The other rapidly turns colourless. A third sample is ignited and produces a sooty flame.

Before reading on, predict what kind of hydrocarbons could produce each observation. What does each test tell you about the bonding inside the molecule?

Learning Intentions

goals

Know

Key facts

The conditions and products for combustion, substitution, and addition reactions
Why alkenes decolourise bromine water but alkanes do not
How addition polymerisation forms long-chain polymers from monomers

Understand

Concepts

Why the C=C double bond makes alkenes more reactive than alkanes
Why incomplete combustion gives harmful products such as CO and soot
How to distinguish substitution from addition in exam questions

Can do

Skills

Write balanced equations for hydrocarbon reactions
Predict reaction type from molecular structure and reagents
Explain tests and observations using bond-level reasoning

Scan these before reading

vocab

combustionReaction with oxygen that releases energy. Hydrocarbon combustion produces carbon oxides and water.

substitution reactionOne atom or group is replaced by another. Typical of alkanes reacting with halogens under UV light.

addition reactionAtoms add across a multiple bond, converting an unsaturated molecule into a more saturated one.

monomerA small repeating molecule that joins to other monomers to form a polymer.

polymerA large molecule made of many repeating units covalently joined together.

unsaturated hydrocarbonA hydrocarbon containing at least one C=C or C≡C bond, so it can undergo addition reactions.

📚 Core Content

Which Hydrocarbon Does What?

A fast way to organise this lesson is to stop memorising isolated equations and start classifying reactions by the bond being targeted. All hydrocarbons combust. Alkanes mainly substitute. Alkenes mainly add.

Alkanes

Single bonds only. Do not readily add bromine, but can undergo substitution with halogens when UV light initiates the reaction.

Alkenes

The electron-rich C=C bond is the reactive site. Bromine, hydrogen, hydrogen halides, and steam can add across it.

Combustion

All hydrocarbons burn in oxygen. Full oxygen → CO₂ + H₂O. Limited oxygen → CO and/or soot.

Polymerisation

Alkenes can repeatedly add to each other. The double bond opens and forms a long carbon chain polymer such as polyethene.

Exam Rule: Start by identifying the hydrocarbon class. If the molecule contains only C-C single bonds, expect combustion or substitution. If it contains a C=C bond, addition reactions and bromine-water decolourisation become available.

Quick check: Which hydrocarbon class mainly undergoes substitution with halogens under UV light?

Explain: oxygen supply controls combustion products

Complete vs Incomplete Combustion

Combustion is the most economically important hydrocarbon reaction, but it is also the one students answer too vaguely. Good responses distinguish complete from incomplete combustion, state the products, and explain the environmental or health consequences of each.

With excess oxygen, a hydrocarbon undergoes complete combustion to form carbon dioxide and water. With limited oxygen, incomplete combustion forms carbon monoxide and/or solid carbon particles (soot) as well as water.

Condition	Main products	Observation	Impact
Complete combustion	CO₂ + H₂O	Cleaner blue flame	CO₂ contributes to greenhouse warming
Incomplete combustion	CO + H₂O and/or C + H₂O	Yellow luminous sooty flame	CO is toxic; soot damages lungs and air quality

Common Error: Do not write "incomplete combustion makes less carbon dioxide so it is better." Incomplete combustion is dangerous because it produces carbon monoxide and particulates. Lower CO₂ alone does not make it safer or cleaner.

HSC chemistry comparison of complete vs incomplete hydrocarbon combustion migrated to comparison-table.

Same fuel, different oxygen supply, different products.

True or false: Incomplete combustion is safer than complete combustion because it produces less CO₂.

Represent: saturated hydrocarbons substitute

Substitution of Alkanes with Halogens

Alkanes are relatively unreactive because they contain only strong sigma bonds and no electron-rich multiple bond. When they do react with halogens such as chlorine, the reaction is a substitution: one hydrogen atom is replaced by a halogen atom.

The classic HSC example is methane reacting with chlorine under UV light. The UV energy breaks the Cl-Cl bond and initiates a free-radical chain process.

top-band Method

How to Identify Substitution Fast

1. Check whether the hydrocarbon is saturated.

2. Look for a halogen reagent such as Cl₂ or Br₂ with UV light.

3. Compare reactant and product formulas: one H is gone, one halogen has replaced it.

4. Name the by-product: hydrogen halide such as HCl or HBr.

Reactant

Methane

CH₄ is a saturated alkane with only C-H and C-C single bonds.

Reagent

Chlorine

Cl₂ supplies the halogen atom that will replace one hydrogen atom.

Condition

UV Light

Ultraviolet light provides the energy needed to initiate the substitution process.

Products

Chloromethane + HCl

The hydrocarbon framework remains intact while one hydrogen is replaced.

Common Error: Do not call methane + chlorine an addition reaction. Nothing adds across a multiple bond because methane does not have one.

Complete: In the substitution reaction between methane and chlorine, the UV light acts as an _____ source (not a catalyst), and the by-product formed alongside chloromethane is _____.

Respond: unsaturated hydrocarbons add

Addition Reactions of Alkenes

The double bond is the reactive feature that distinguishes alkenes from alkanes. In an addition reaction, the pi bond is broken and new single bonds are formed to the added atoms or groups.

Reagent	Example with ethene	Product	Use in HSC
Br₂	C₂H₄ + Br₂	1,2-dibromoethane	Test for unsaturation
H₂	C₂H₄ + H₂	Ethane	Hydrogenation
HCl / HBr	C₂H₄ + HCl	Chloroethane	Haloalkane formation
H₂O (steam)	C₂H₄ + H₂O	Ethanol	Hydration pathway

Key Test: Bromine water decolourises in the presence of an alkene because Br₂ adds across the C=C bond. An alkane under normal classroom conditions does not rapidly decolourise bromine water.

Worked Example 1

Reaction Type

Problem: Ethene is bubbled through bromine water. Explain the observation and write the equation.

Ethene is an alkene, so it contains a reactive C=C bond.

Bromine adds across the double bond, so the orange bromine colour disappears.

The balanced equation is C₂H₄ + Br₂ → C₂H₄Br₂.

Answer: Bromine water decolourises because bromine adds across the C=C bond in ethene. This is an addition reaction: C₂H₄ + Br₂ → C₂H₄Br₂.

Odd one out: Which of these is NOT an addition reaction of an alkene?

Addition Polymerisation

Addition polymerisation is repeated addition chemistry. Each alkene monomer opens its double bond and links to neighbouring monomers, creating a long carbon chain. The atoms of the monomer all remain in the polymer repeating unit.

For ethene, the polymer is polyethene. The monomer is CH₂=CH₂. The repeating unit is written as [−CH₂−CH₂−]_n. Brackets show the repeating pattern and n shows that the unit repeats many times.

Monomer and repeating unit must not be confused in exam responses.

Monomer

A small alkene molecule before reaction, for example ethene.

Repeating unit

The structural pattern inside the polymer chain shown in brackets.

Polymer

The complete macromolecule made of many repeating units joined together.

Critical Error: Do not write the polymer of ethene as simply "(CH₂=CH₂)_n". The repeating unit must show that the double bond has opened: [−CH₂−CH₂−]_n.

Worked Example 2

Polymerisation

Problem: Draw or describe the repeating unit formed when propene polymerises.

Start with the monomer: CH₂=CHCH₃.

The double bond opens during addition polymerisation.

The carbon skeleton remains, so the repeating unit becomes [−CH₂−CH(CH₃)−]_n.

Answer: The repeating unit is [−CH₂−CH(CH₃)−]_n. The CH₃ side group remains attached to every second carbon in the polymer chain.

What is the correct repeating unit for the addition polymer of ethene?

Feedback: select the reaction from bonds and evidence

Pick the Reaction Type

Use this sequence whenever a question gives you a structural formula and a reagent list.

Decision Sequence

Step 1: Identify whether the organic molecule is saturated or unsaturated.

Step 2: Identify the reagent and any condition such as UV light or oxygen supply.

Step 3: Match structure + reagent to reaction family: combustion, substitution, or addition.

Step 4: Predict the product by tracking which bond breaks and what atoms are added or replaced.

Must Do: In HSC extended responses, do not stop at naming the reaction. State the specific bond feature responsible for the reactivity, such as "the alkene's C=C bond undergoes addition".

Worked Example 3

Equation Writing

Problem: Write the complete combustion equation for butane, C₄H₁₀.

Write the skeleton: C₄H₁₀ + O₂ → CO₂ + H₂O.

Balance C first: 4 carbons means 4CO₂.

Balance H next: 10 hydrogens means 5H₂O.

Count oxygen atoms on the right: 8 + 5 = 13 O atoms, so use 13/2 O₂, then multiply through by 2.

Answer: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.

Match the reaction to its type: (1) C₂H₄ + Br₂ → C₂H₄Br₂ | (2) CH₄ + Cl₂ + UV → CH₃Cl + HCl | (3) CH₄ + 2O₂ → CO₂ + 2H₂O | (4) nCH₂=CH₂ → [-CH₂-CH₂-]ₙ

Beyond the syllabus. Reaction classification, combustion, substitution and basic addition polymerisation are core. Fractional distillation, cracking and free-radical mechanism detail are extension — the exam tests the reaction families, their conditions and observations.

Implications of Obtaining and Using Hydrocarbons

NESA syllabus dot point: examine the environmental, economic and sociocultural implications of obtaining and using hydrocarbons from the Earth. The command word examine asks you to inquire into and weigh several perspectives, not just list facts.

We obtain hydrocarbons from the Earth as crude oil, natural gas and coal seam gas, then separate crude oil into useful fractions by fractional distillation. Those fractions are burned as fuels (petrol, diesel, LPG, kerosene) and used as feedstock to make plastics, solvents, detergents and medicines. Because hydrocarbons sit at the centre of modern energy and manufacturing, obtaining and using them carries implications that reach well beyond chemistry.

Environmental

Complete combustion of hydrocarbons releases carbon dioxide, a greenhouse gas that contributes to climate change; incomplete combustion adds carbon monoxide and soot. Extraction and transport can cause oil spills and habitat disruption, and hydrocarbons are a finite, non-renewable resource that cannot be replaced on a human timescale.

Economic

Hydrocarbons provide cheap, energy-dense fuel that underpins transport, electricity and industry, and they are a major source of jobs and export income for Australia through liquefied natural gas and coal. Against this, global prices are volatile, and moving to lower-emission alternatives carries a real transition cost.

Sociocultural

Access to affordable energy raises living standards and shapes how communities live and work. Extraction projects can also affect Aboriginal and Torres Strait Islander communities and Country, including cultural heritage sites and land use, which is why consultation and land rights are central to the debate. Society must also weigh the needs of today against intergenerational equity for future generations.

Exam technique: to examine the implications, present both benefits and costs across the environmental, economic and sociocultural dimensions, then reach a supported judgement rather than a one-sided answer.

Which response best examines the implications of using hydrocarbons, as the syllabus command word requires?

High-Frequency Errors to Avoid

"Bromine water tests for all hydrocarbons." Wrong. It is primarily a test for unsaturation, especially alkenes.

"Substitution and addition both just mean chemicals react together." Wrong. Addition opens a multiple bond; substitution replaces one atom or group with another.

"The repeating unit is the same as the monomer." Wrong. The repeating unit shows the structure after the C=C bond has opened and linked into the chain.

→

Key Reactions

C_xH_y + O₂ → CO₂ + H₂O Complete combustion · Excess oxygen

CH₄ + Cl₂ → CH₃Cl + HCl Substitution · UV light required

C₂H₄ + Br₂ → C₂H₄Br₂ Addition across C=C · Bromine water decolourises

nCH₂=CH₂ → [−CH₂−CH₂−]_n Addition polymerisation · Ethene → polyethene

Bonding decides reactivity: single bonds only usually substitute; double bonds add; all hydrocarbons combust.

Interactive Tool, Hydrocarbon Reactions Predictor Open fullscreen ↗

True or false? Alkanes react with halogens by substitution under UV light, while alkenes react by addition across the C=C double bond.

🔬 Predict, Then Reveal +8 XP

Predict: when propane burns in a limited oxygen supply (incomplete combustion), what carbon-containing products form and why don't they form CO₂?

Your predictionExpert answerCompare

Apply: equations, observations and implications

Complete the Learn phase to unlock Practice.

✏️ Activities

Classify from Evidence

A chemist carries out four experiments and records the observations below. For each experiment, identify the reaction type and justify your answer using the evidence.

Observation

A. Bromine water stays orange.

B. Bromine water rapidly decolourises.

C. Clean blue flame; CO₂ and water vapour detected on a cold surface.

D. The product tests positive for an OH group.

Reaction type + justification

Organic Compound Identification

For each compound or description below, identify the functional group, name the compound using IUPAC nomenclature, and draw its structural formula.

A compound with the molecular formula C₅H₁₂O reacts with acidified dichromate to produce a ketone. Identify the functional group, name the compound, and draw its structure.
An alkene with the formula C₄H₈ undergoes hydrogenation to form butane. Draw the structure of the original alkene and name it.
Describe the difference between complete and incomplete combustion, including one example of each.

📝 Check Your Understanding

Randomised Question Bank

Complete five questions drawn from the ten reactions items aligned to this lesson.

Exam-Style Practice

1. Explain why bromine water can distinguish between ethane and ethene. 3 MARKS

2. Propene reacts with steam to form an alcohol. Identify the reaction type and explain the bond changes that occur. 4 MARKS

3. A yellow smoky flame is observed when a hydrocarbon burns in a limited oxygen supply. Explain what this observation suggests about the reaction products and why those products are concerning. 5 MARKS

Show All Answers

Activity 1, Classify from Evidence

A. No reaction (or very slow substitution only if UV is present). Hexane is saturated, it has no C=C bond, so bromine cannot add across a double bond. Without UV light, substitution is not initiated either, so the orange colour remains.

B. Addition reaction. Hex-1-ene contains a C=C bond. Bromine adds across the double bond to form a dibromoalkane, consuming Br₂ and removing its orange colour.

C. Complete combustion. Excess oxygen means all carbon is fully oxidised to CO₂ and all hydrogen to H₂O. The clean blue flame and absence of soot confirm complete combustion.

D. Addition (hydration). Ethene reacts with water (steam) across its C=C bond to form ethanol. The OH group detected confirms an alcohol product formed by addition of water across the double bond.

Multiple Choice

1. Correct response: Rapid bromine-water decolourisation is the clearest sign of a C=C bond and unsaturation.

2. Correct response: A hydrogen atom on methane is replaced by chlorine, so the reaction is substitution.

3. Correct response: Incomplete combustion forms partially oxidised carbon products such as CO and soot.

4. Correct response: Addition polymers form when alkene double bonds open and join into long chains.

5. Correct response: H and Cl add across the double bond, so the reaction is addition.

Short Answer Model Answers

Q1 (3 marks): Ethene contains a carbon-carbon double bond [1]. Bromine adds across this double bond, so bromine water is decolourised [1]. Ethane is saturated and has no C=C bond, so it does not rapidly react with bromine water under normal conditions [1].

Q2 (4 marks): This is an addition reaction [1]. Propene contains a reactive C=C bond [1]. During hydration, the double bond opens and the atoms of water add across the bond [1]. The product is an alcohol because an -OH group is introduced into the molecule [1].

Q3 (5 marks): A yellow smoky flame suggests incomplete combustion due to limited oxygen supply [1]. Instead of all carbon atoms forming CO₂, some form carbon monoxide and/or solid carbon soot [1]. Carbon monoxide is dangerous because it binds strongly to haemoglobin and reduces oxygen transport in the blood [1]. Soot particles are also harmful because they contribute to respiratory disease and poor air quality [1]. The flame appears smoky because glowing carbon particles are present [1].

Callback, The Bromine Water Hook Resolved

Back at the start, you were shown two colourless gases bubbled into separate tubes of bromine water, one tube stayed orange and the other instantly turned clear. Now you know exactly why.

The gas that decolourised the bromine water is an alkene (or alkyne, any unsaturated hydrocarbon). It contains a reactive C=C double bond. Bromine adds across the double bond in an addition reaction, forming a dibromoalkane and consuming the bromine, which is why the orange colour disappears: C₂H₄ + Br₂ → C₂H₄Br₂.

The gas that stayed orange is an alkane. Alkanes contain only C–C and C–H single bonds, no pi bond for bromine to add across. Without UV light to initiate free radical substitution, bromine does not react with alkanes under normal conditions. The colour remains.

The bromine water test is therefore a test for unsaturation: decolourisation confirms a C=C (or C≡C) bond is present. An alkane simply does not provide the reactive site that makes addition possible.

Review: retrieve the reaction logic

Check what actually stuck

🏆 Review

What are the products of complete combustion of a hydrocarbon?

What observation confirms that an alkene is present when bromine water is used?

Why is UV light described as an "energy source" rather than a "catalyst" in alkane halogenation?

What is the repeating unit for the addition polymer of propene?

What are the health hazards of incomplete combustion products?